Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Civil Engineering (CE) : Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

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Failure Investigation of A Prestressed Concrete Bridge Girder
 Objective: 
To investigate the failure of a prestressed girder in accordance with ACI 318-02.

Problem: A highway overpass consists of 3 parallel continuous prestressed concrete beams. The length of the overpass structure is 292.8 ft, with a width of 47 ft (Fig. 1 and 2). Each prestressed beam had 5 strands of prestressing steel. There were 22 wires in each strand and each wire had a diameter of 0.6 in. The end of each prestressed beam was supported by a corbel, which was inclined at an angle with respect to the bearing plate (Fig. 3, 4, and 5).
Construction proceeded as planned: the beams were cast-in-place, and after the concrete hardened, they were post-tensioned. Minutes after the prestressing operation, 4 out of the 6 corbels broke (Fig. 3). The State Transportation Authority decided to determine the responsible parties involved in this failure case.

Task: You are hired to be the expert witness on the case. The following information were established:
(a) The reaction force (R) at each end of the beam right before the collapse was estimated at 275 kips.
(b) The horizontal restraint offered by the bearing (i.e., the Teflon disk) is negligible.
(c) Normal weight concrete was used with the compressive strength of fc ’ = 5000 psi.
(d) Yield stress for normal reinforcement was fy = 60 ksi.

Using the above information and the attached drawings, you are asked to assess and testify on the following questions:
(1) Was the design (Fig. 6) adequate in accordance with ACI code requirements?
(2) It was reported that the elastic shortening of the beam due to the initial prestressing was 0.9 in (Fig. 7). Check the design adequacy for this situation.
(3) It is postulated that the workmen might have placed the Teflon disk in the wrong position initially. Together with the elastic shortening due to prestressing, the final position of the Teflon disk was as shown in Fig. 8. Check the design again using the ACI code.
(4) Based on the above information, give your opinion as to the cause(s) of the collapse. It was argued that if instead of having the corbels, the prestressed beams were cast into the piers as a whole unit (i.e., fixed ends), and then the failure would not have occurred. Do you foresee any problems with this design?

Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Figure 1. Plan view of overpass

Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
Figure 2. Cross section of overpass (section a-a)

Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
Figure 3. Location of failure (section b-b)

Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
Figure 4. End zone detail for prestressed beam (section c-c)


Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
Figure 5. Plan view of end zone (section d-d)

Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Figure 6. Elevation view of corbel (section e-e) (Design Drawing)

Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
Figure 7. Elevation view of corbel (section e-e) (after initial prestressing, elastic shortening at each end of the beam, ∆L)

Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
Figure 8. Elevation view of corbel (section e-e) (Postulate failure configuration)

(I) Engineering Drawing:                                                                                                                       ACI
Load on the corbel
– reaction                                      R = 275 kips
– factored reaction                        Ru = 1.4 R = 385 kips
– area of Teflon disk                    Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
– uniform stress on the Teflon disk Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
– shaded area                                A’ = 44.55 in2
– shear force                                  Vu = σu ⋅A ’ = 151.5 kips
– tension                                        Nuc = 0 kips                                  11.9.3.4
– moment                                      M= Vu ⋅ a                                    11.9.3.2
                                                           = 151.5 x 2.45
                                                           = 371.2 kips-in

Corbel dimension 
                                             Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev                        

Shear design 
                                       Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
                                                  
                                                      Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Flexural design

                                        Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Tension design

                                                 Since Nuc = 0                                                11.9.3.4
                                                           A= 0


Primary tension reinforcement 
                                                 
                                                 As = A+ An = 0.383 in2                               11.9.3.5
                                            or Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

From the design, there are 4 - #6 bars provided. 
                                             
                                                  (As)provided = 4 0⋅ .44 in2
                                                                     = 1.76 in2 > 1.41 in2 (O.K.)

Ties 

                                            Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

From the design, over Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev = 15.33 in , there are 3 - #5 bars provided.

                                      Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev


Reinforcement ratio
                                           

                                     Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
                                             
⇒ The engineering design in Fig. 6 is adequate in accordance with ACI code 318-02.

(II) With elastic shortening
 Similarly, we have


                                         Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Corbel dimension 

                                        Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Shear design 

                                  Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev


Flexural design 

                                    Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
                                   Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev


Reinforcement ratio 
                                   Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

⇒ The engineering design in Fig. 6 is adequate in accordance with ACI code 318-02.

(II) With elastic shortening
 Similarly, we have 


                                        Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Corbel dimension 
 

                                Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

Shear design
                                 
                                     Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev
Flexural design

                                    
Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev


Primary tension reinforcement 
                                  
                             Failure Investigation of A Prestressed Concrete Bridge Girder Civil Engineering (CE) Notes | EduRev

⇒ With misplacement of Teflon disk and elastic shortening, the given design is not adequate.

(IV) There is a good chance that the failure was due to poor design or inadequate considerations on the part of engineer. Even if the Teflon disk was correctly placed, with the elastic shortening of beam and live loads, the bridge does not have much of a chance of surviving. Misplacement of the Teflon disk greatly increased the risk of failure since no information on the site supervision on the pat of the engineer was given. A probable cause of the failure could then be attributed to both the engineer and the contractor.

If the beam is cast monolithically into the pier, problems that might arise are
– Secondary stresses induced die to creep, shrinkage and elastic shortening;
– Thermal stresses created due to differential temperature effect.

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